Oxofluoride complexes

In a milder variant, fluoride anions are used as mineralizers [23], A typical synthesis system then consists of reactive silica, the SDA, water and NH4F or HF as fluoride source. The pH can be adjusted to neutral or to slightly acidic. It is presumed that the silica is mobilized as fluoride or oxofluoride complexes. Fluoride-mediated syntheses usually yield compounds of high crystallinity, but the crystallization times are longer than for alkaline preparations. 

The presence of oxides and the formation of oxofluoro-complexes in molten electrolytes may be sometimes unwanted, but in many cases they are the fundamental features of the system. For instance, the formation of oxide complexes in alkali-alkaline earth chloride melts may be mentioned. The formation of oxofluoride complexes in molten cryolite-alumina melts, used as electrolytes for aluminum production, is typical as well. On the other hand, the presence of oxofluoride complexes in electrolytes used for niobium production was initially regarded as unwanted. Recently, however, it has been proven that their presence in niobium electrolytes plays an important role in the niobium electrodeposition. In the following, some technologically important examples of systems containing halides and oxides will be described. 

Kuznetsov, S.A., and Grinevich, V.V. (1994) Niobium interaction with its chloride, fluoride and oxofluoride complexes in melts of alkali chlorides, Zh. Prikl. Khim 67,1423-1430. 

Ta(V) and B(III) possess a stronger complexation potential, than the alkali cation thus the oxofluoride complexes of tantalum and boron predominate in oxide-ion... 

Kuznetsov, S.A. and Glagolevskaya, A.L. (1995) The electrochemical behaviour of tantalum oxofluoride complexes in an equimolar mix of potassium and sodium chlorides, Tr. VIII Kola Seminar on Electrochemistry. KSC RAS. Apatity, 44... 

For Se only Se02F2 is known. For Te there are no Te02X2 compounds, but recently TeOF2 and Te have been reported. The structure of TeOF2 is unknown but Te d has a complex polymeric structure. Both selenium and tellurium, as well as sulfur, form a number of more complex oxofluorides, a few of which are shown, with their structures, where known, in Table 12-7. 

The presence of oxide in the melt causes the formation of various niobium oxofluoro-complexes. It was shown that relatively pure Nb coatings can be obtained from FLINAK melts if no/wNb is less than 1. However, it has been shown by Konstantinov et al. (1981) and Khalidi et al. (1991) that, even a small amount of 0 ions can entirely change the mechanism of Nb deposition depending on the types of the niobium oxofluorides formed in the melt. 

Assuming that Fe203 reacts with cryolite under the formation of fluoride and oxofluoride aluminum complexes according to the equation... 

Anionic Species Containing the 149 + Unif. Complex oxofluorides are well established, as are salts of [VOCL]" and... 

We have studied the electrochemical behaviour of chloride, fluoride, and oxofluoride compounds of rhenium and established the conditions for the stabilization of Re(III), Re(IV), Re(VI), and Re(VII) complexes. For a more rigorous elucidation of interrelations between the redox potentials, the composition of melts, and the oxidation state of rhenium, it is necessaiy to undertake spectroelectrochemical investigations. Rhenium coatings with a thickness up to 2 mm on graphite and noble metal substrates were produced. 

No complex salts or molecular adducts derived from the known oxofluorides and oxide of xenon in the -1-8 oxidation state, namely, Xe03F2, Xe02F4, and Xe04, are known. 

---